This invention relates to quartz crystal microbalances of the type used in measuring film deposition thickness and deposition rates. The invention is more particularly concerned with an improved piezoelectric crystal construction that prolongs the crystal's useful life.
Quartz crystal microbalances or quartz crystal deposition monitors are widely employed in film deposition work where the thickness and rate of deposition have to be closely monitored. The thickness shear mode resonance of an AT-cut piezoelectric quartz crystal is very useful in monitoring the rate of deposition of thin solid film on a substrate. In a typical arrangement, a sensor quartz crystal with appropriate electrodes is placed in the deposition chamber next to the substrate to be coated and is exposed to the material vapors. The materials condense on the face of the crystal at a rate that can be easily monitored with a suitable oscillator driver circuit. There is a direct proportion relationship between the amount of material deposited on the face of the quartz crystal and that deposited on the substrate. This proportionality constant, called the tooling factor, depends on the physical locations of the crystal and the substrate with respect to the source. Thus, the amount of foreign mass deposited on the substrate can be deduced from changes in the resonant frequency of the crystal. This is the essence of the working of a quartz crystal microbalance (QCM) instrument.
There are a number of methodologies available for interpreting the frequency-shift to mass-deposit relation. Most recently, QCM instruments have relied on the well-known Lu-Lewis formulation or on a so-called Z-match technique. These techniques have improved the sensitivity of measurement, to the limit of the ability of the crystal to receive continued deposition without resonance failure.
Some important applications QCM instruments involve vacuum deposition of metals and dielectrics by physical vapor deposition or sputtering methods in the semiconductor and the optical coating industry. To a lesser extent, these devices are also used in MBE, CVD and electroless plating operations.
AT-cut quartz crystals are the primary choice as sensor crystals for QCM instruments. This particular cut of crystal has the merit of superior mass sensitivity, significantly reduced temperature coefficient and relatively low manufacturing cost. For this type of crystal, the dominant mode of vibration is thickness shear, although some degrees of thickness twist is also present. Thus, various combinations of thickness shear and twist motion give rise to numerous resonances. The lowest frequency resonant mode is called the fundamental mode and is of primary importance in the context of the QCM instruments. Anharmonics and quasiharmonics may also be of interest, but these accompany the fundamental in a predictable manner.
Sensor crystals tend to be discs or discoids, although other shapes are also possible. Typical diameters of the sensor crystals range between 0.5 to 1.0 inch. For performance related reasons, there are restrictions on the diameter-to-thickness ratio of the crystal blanks, particularly for the plano-plano crystals. Such crystals have their principal surfaces plane and a parallel to each other. However, the majority of sensor crystals belong to a category called plano-convex. Such crystals have one of the principal surfaces rendered convex. The process is called contouring and it results in superior performance due to an effect called energy trapping (confinement of vibrational energy in the central region).
In order to excite and detect resonance vibrations in a piezoelectric quartz crystal, appropriate electrodes must be placed on its surfaces. Typically, one face of the crystal is completely coated with a chosen electrode material. On the other face (usually the contoured one) a smaller electrode is placed, just large enough to cover the active area.
The electrode material is usually metallic, such as aluminum, silver or gold. Generally, electrode thicknesses are 2,000-5,000 .ANG.. If gold or silver is used, an underlayer of titanium or chromium (50-300 .ANG.) is also applied prior to electrode placement, in order to increase the electrode adhesion to the quartz plate. Generally, aluminum and silver coated crystals have shorter shelf life, possibly due to surface oxidation or sulphur contamination.
The sensor crystals to be used with QCM instruments have a limited lifetime. That is, the crystals are effective only for a limited thickness of material deposited thereon. As material is deposited upon a crystal, its quality (resonator Q-factor) progressively degrades. When it deteriorates to such a point that the resonance is no longer well defined, then the driving/detecting circuitry fails to detect the desired resonance. At that point, the crystal is no longer useable. At what stage a crystal will fail depends on many factors, such as the initial quality of the sensor crystal (measured in terms of its starting motional resistance), the characteristics of the deposition material, the rate of deposition, ambient temperature, etc. In general, in high temperatures and at high deposition rates, the crystal is likely to fail earlier for a given material. There is a dramatic difference in resonance sharpness for the fundamental mode resonance of an uncoated and a coated crystal.
For a typical 6 Mhz sensor crystal, the useful life could be more than 2 Mhz of frequency shift for materials like copper. In such materials, the failure occurs due to internal friction (viscous damping) in the thick film of the deposited material. For materials like nickel, the life could be less than 0.7 Mhz of frequency shift, even at low deposition rates. Here, the failure mechanism is different. Because nickel film has high intrinsic stress, it literally tears itself apart when it grows to a sufficient thickness, taking part of the quartz with it. When two dissimilar metals are deposited in alternate order (e.g., in optical coatings or engineered band structures), lattice mismatch and differences in thermal expansion coefficients rapidly increase stress in the interface, eventually causing total loss of resonance.
One of the important applications of QCM instruments is in optical coatings. Various dielectric materials, such as magnesium fluoride, silicon dioxide, titanium dioxide and cryolite are deposited on lenses, mirrors and eyeglasses. The aim is to fabricate optical bandpass filters and anti-reflection coatings over some wavelength spectrum of interest. These coatings are deposited at very low and precise rates to control the optical characteristics of the final product. In the semiconductor industry, insulating silicon dioxide and other dielectrics are often deposited on silicon substrates as diffusion barriers, although precise rate may be less critical.
The quartz crystal microbalance is the only practical means for in situ monitoring and control of the growth rate of such optical/dielectric films. Other methods, such as optical interferometry and ellipsometry, are cumbersome and expensive. Low energy electron impact induced emission spectroscopy (EIES), though highly successful in metal depositions, is insensitive to dielectric materials.
It commonly occurs that when a quartz crystal is used for monitoring the deposition of dielectric material, the useful crystal life is drastically shorter than for the same crystal to monitor the deposition of metals. Some materials, like magnesium fluoride and zirconium dioxide, brutally shorten crystal life. For such materials, the crystal ceases to operate at only about 0.30 Mhz of frequency shift into the process. To make matters worse, crystal failure occurs rather suddenly, and the resonator Q-factor rapidly deteriorates. For many applications, this is a serious hindrance. One way to overcome this handicap is by having a reserve of multiple crystals within the vacuum chamber. But even then the operator must detect the advent of an imminent crystal failure and promptly switch over to a new crystal, if available. Otherwise the process will suffer. In contrast, metal deposition degrades crystal quality rather progressively, giving forewarning to change the crystal in order to save the work in process.
Two major factors contribute to the early demise of sensor crystals in the dielectric depositions. First, an excessively smooth surface of a high-Q quartz resonator lacks sufficient keying in points. That is, a highly polished crystal lacks surface roughness, which does not help the cause of adhesion of the deposited film. Sometimes, when deposition is at a high rate, the film simply peels off from the surface of a crystal due to this smoothness coupled with built up stress in the deposited film. This phenomenon is known as "snowing" because of the appearance of small flakes on the electrode surface.
Second, and more importantly, dielectric films generate high interfacial stress but have little mechanical strength. It is observed that, when the deposited film's thickness becomes appreciable, the quartz crystal's Q-factor dramatically diminishes and it is no longer useful as a mass sensing mechanism. In the deposition monitor instruments, the familiar signature of this failure is a rapid increase in the apparent or measured deposition rate (or decrease of frequency) followed by loss of resonance within a very short period of time.
Thus, the superficial symptoms of crystal failure in the optical coating processes appear to be different. When a crystal with dielectric coating is examined under a metallurgical microscope, the grains on the surface have a non-uniform appearance. This suggests the growth to be columnar in nature. The low adhesion of some dielectric films to the gold or silver electrodes further complicates this problem and further hastens failure.
It is believed, at least by the inventor here, that crystal failures in the depositions of dielectric films are due to stress distortion in the deposited layer. In the case of one particular failed crystal with a silicon dioxide deposit on it, it was found that the surface has broken at random into many small but distinctly continuous pieces. The situation is similar to the breakage of a thin sheet of ice on a frozen pavement, when one walks over it. When stress (intrinsic and/or thermal) in the film became excessive, it attempted to distort the quartz crystal to relieve stress. The crystalline quartz blank, being much thicker and stronger, resisted distortion. As a result, the film itself crumbled under the dint of its own stress.
As mentioned before, the acoustic loss mechanism in most metals is due to internal friction, that is, due to viscous losses within the bulk of the material. In such case, the equation of wave propagation involves one damping parameter, .alpha., as embodied in the equation EQU A(x)=A.sub.0 cos(kx-wt) exp(-.alpha.x).
Here, A(x) is the amplitude of wave at some distance, x, along the propagation direction from a point where the amplitude is A.sub.0. The parameters k and w represent wave number and angular frequency, respectively. The attenuation coefficient, .alpha., remains relatively unchanged. Therefore, the longer the acoustic path (i.e. thicker the deposited film), the more the acoustic energy loss (proportional to the square of amplitude). This conforms to the general observation of the slow degradation of crystal resonances in metal depositions. In contrast, the acoustic loss mechanism in the optical/dielectric films is largely due to surface scattering.
In a quartz crystal or any other similar mechanical resonator, the phenomenon of resonance exists because of constructive interference of the reflected waves from the two principal surfaces. This is possible because both of the reflected wave fronts are coherent, as the reflecting surfaces are generally smooth and continuous. On the other hand, if one of the surfaces is very rough or discontinuous, the reflected wavefront is incoherent, because different parts of the wavefront undergo different amount of phase shifts. When such reflected waves interfere with each other, the general result is the loss of resonance. Observed rapid failures in the case of dielectric coatings support this theory.